1. Field of the Invention
This invention relates to photoinitiator compositions useful in photopolymerizable compositions. It also relates to photopolymerizable compositions containing the photoinitiator compositions of this invention and to a process for making a photoresist which employs a photopolymerizable composition containing the photoinitiators of this invention.
2. Description of the Prior Art
Photoinitiators have long been used in photopolymerizable compositions to initiate photopolymerization upon exposure of the composition to actinic radiation, and to increase the speed at which the polymerization takes place.
Various compounds have been employed as photoinitiators. For instance, D. P. Specht et al. disclose in Tetrahedron, Vol. 38, No. 9, pages 1203 to 1211, 1982 in an article entitled "Ketocoumarins-A New Class of Triplet Sensitizers" that 3,3'-carbonyl biscoumarins, including the 7,7'-dialkylamino derivatives thereof, are efficient triplet sensitizers for photopolymerizable compositions. They further disclose that these compounds are useful in photoresist films and are very efficient in the 480-500 nm range, i.e. in the visible laser range.
In U.S. Pat. No. 4,289,844, issued Sept. 15, 1981, Specht et al. disclose a co-initiator system for photopolymerizable compositions which contains a carbonyl compound and an activator. The carbonyl compound is a 3-ketocoumarin, e.g. 3,3'-carbonylbis (7-diethylaminocoumarin). The activator may be selected from a variety of compounds such as amines other than a 3-ketocoumarin amine, acetic acid derivatives, phosphines, phosphites, bismuthines, arsines, stibines, sulfinic acid and sulfinic acid esters, sulfones, .alpha.- and B-dicarbonyls, ketones, bi-imidazoles and stannates. Examples of activators include 2,3-bornanedione (camphorquinone) and alkyl esters of p-dialkylamino benzoic acid. It is further disclosed that mixtures of activators may be employed, though no specific combinations are disclosed. However, Specht et al. do caution that not all free radical photopolymerization activators will work with their courmarins to provide the desired result. Some combinations are said to fail to achieve the desired speed.
U.S. Pat. No. 4,147,552 to Specht et al., issued Apr. 3, 1979, also discloses light sensitive compositions which employ 3,3'-carbonyl bis(7-diethylaminocoumarin) as a photosensitizer. This compound may be used in combination with other sensitizers, 3-benzoyl-7-methoxycoumarin being disclosed as the only example of such co-sensitizers. The 3,3'-carbonyl bis(7-diethylaminocoumarin) is stated to be of particular interest when lasers are used as the light source.
Specht et al., in U.S. Pat. No. 4,278,751 issued July 14, 1981, disclose a co-initiator system for a photopolymerizable composition. The co-initiator contains an amine-substituted ketocoumarin, such as 3,3'-carbonyl bis(7-diethylaminocoumarin), and an activator such as N-phenylglycine or N-methylindoleacetic acid. A comparative example (Example 9) employs 3,3'-carbonyl bis(7-diethylaminocoumarin) and ethyl p-dimethylaminobenzoate as the co-initiator.
U.S. Pat. No. 4,529,491, issued July 16, 1985 to Hauw et al., discloses photopolymerizable compositions which use benzil and/or fluorenone, a biphenylyl ketone and a reducing agent as the photosensitizer. The reducing agent may be 4-dimethylamino benzoate.
In U.S. Pat. No. 3,756,827, issued Sept. 4, 1973, Chang discloses photopolymerizable compositions containing a photosensitizer system which employs a cyclic cis .alpha.-dicarbonyl compound and an actinic radiation absorbing compound. The dicarbonyl compound may be 2,3-bornanedione (camphorquinone) and the actinic radiation absorbing compound can be a 7-dialkylamino-4-alkyl coumarin.
Biimidazoles (or imidazolyl dimers) have also be used in photoinitiating systems for photopolymerizable compositions. U.S. Pat. Nos. 3,479,185 (issued Nov. 18, 1969 to Chambers); 3,549,367 (issued Dec. 22, 1970 to Chang et al.) and 4,252,887 (issued Feb. 24, 1981 to Dessauer) disclose photoinitiators containing triarylimidazolyl dimers. These compounds also find utility in photosensitive, color forming compositions such as those disclosed in U.S. Pat. Nos. 3,784,557 (issued Jan. 8, 1974 to Cescon); 3,445,233 (issued May 20, 1969 to Cescon); 3,445,234 (issued May 20, 1969 to Cescon et al.); 3,390,994 (issued July 2, 1968 to Cescon); 3,615,481 (issued Oct. 26, 1971 to Looney); 3,658,543 (issued Apr. 25, 1972 to Gerlach et al.) and 4,247,618 (issued Jan. 27, 1981 to Dessauer et al.).
While the photoinitiators described above do provide useful photopolymerization speeds (photospeeds), there is still a need for photoinitiators which provide even faster photospeeds. Also, with the advent of lasers as the light source for the photopolymerization, a need has developed for photoinitiators which will provide photospeeds fast enough that relatively low power lasers can be used to polymerize photopolymerizable materials, for example photoresists, at commercial production rates.